Practical communication systems using light as a signal such as a wavelength-division multiplexed (WDM) optical communication system are known, and longer-distance communication systems having a larger data capacity have been developed.
Recently, in order to increase the capacity, the wavelength-division multiplexed communication which attracts much attention uses advantageous characteristics of light and information signals transmitted via optical signals having a plurality of wavelengths. On the other hand, the direct optical amplification method using an erbium-doped optical fiber amplifier (EDFA) is also practical and transmission distance has rapidly progressed. However, in the optical communication systems using the EDFA, a pulse-shaped optical signal of high intensity may be generated. Therefore, in order to secure high stability and reliability of the system, it is necessary to protect relevant components such as light-receiving elements from damage due to the pulse-shaped high-intensity optical signal. Similar problems may occur in other electro-optic devices not only in the optical communication system.
Various causes with respect to the generation of the extraordinary pulse-shaped high-intensity optical signal are known, such as: (i) when the optical signal input into the system includes a high-intensity optical signal component, or (ii) when the optical signal is amplified in the system, a secondary high-intensity optical signal component is generated. In the optical communication system performing direct optical amplification using the erbium-doped optical fiber amplifier (EDFA), an extraordinary high-intensity pulse-shaped optical signal component may be included in the optical signal which was amplified using the EDFA (refer to T. Imai, et al., Proceedings of "1992 Optical Amplifier Topical Meeting", Presentation No. PD 12, 1992).
The reason for this phenomenon is that Er.sup.3+ which has been excited to a higher level is stored in the EDFA with a high energy while no optical signal is input (that is, when no optical signal exists) in the EDFA, and that when an optical signal is input into the EDFA under the above situation, the stored high energy is rapidly stimulated and emitted so that the emitted portion is added to the input optical signal as an optical surge. Such an optical surge may damage or degrade a light-receiving element connected to the output side of the EDFA. Therefore, it is preferable that such an optical surge be removed.
A method for removing the above-explained optical surge, and also controlling generation of the optical surge in the optical transmission system including the EDFA is known, in which the optical signal transmitter raises the optical signal during a longer period of the "msec" order or more, so as to gradually emit the energy which was accumulated in the EDFA while no signal was being input (refer to Yoneyama, et al., Proceedings of the spring conference of the IEICE, Presentation No. B-941, p. 4-79, 1993).
Another method is known, which considers that the optical surge is generated due to the emission of energy which was accumulated in the EDFA during a non-signal period. In this method, before a target optical signal to be amplified is input into the EDFA, a dummy optical signal having a wavelength different from that of the target optical signal is previously merged with the target optical signal, and the power of the dummy signal is controlled so that the total of the power of the dummy signal and the power of the target signal is fixed, thereby maintaining a fixed amplification gain regardless of the level of the input optical signal. The input optical signal is then amplified using the EDFA without wavelength distortion, and the dummy optical signal is removed from the merged signal by using an optical wavelength-filtering means or the like (refer to Japanese Patent Application, First Publication, No. Hei 6-216452).
As another method similar to the above method, it is known that the wavelength of a dummy signal to be merged with the target optical signal is defined within 1545 to 1565 nm, which is a wavelength transition range of the stimulated emission from a higher level with respect to the EDFA, thereby more effectively depressing the optical surge (refer to Yoneyama, et al., Proceedings of the general conference of the IEICE, Presentation No. B-1190, p. 622, 1996).
As a practical example of the depression of the optical surge by using a dummy signal, a surge prevention circuit for transmitting a dummy signal during interruption of the target optical signal is known (refer to Sato, et al., Proceedings of the general conference of the IEICE, Presentation No. B-1191, p. 623, 1996).
It is necessary in the optical transmission system to prevent or remove the optical surge. All the above-explained conventional methods are to prevent or depress the optical surge which was generated in the amplification process using the EDFA.
In order to prevent or depress the surge, for example, the optical fiber amplifier shown in FIG. 1, which is disclosed in Japanese Patent Application, First Publication, No. Hei 6-216452, comprises many components such as optical coupler 1, dummy signal semiconductor laser emitter 2, dummy signal semiconductor laser emission control circuit 3, light-receiving element 4, optical beam splitter 5, optical multiplexer 6 for multiplexing a dummy optical signal and an excited optical signal, semiconductor laser emitter 7 for excitation, drive circuit 8 for the semiconductor laser emitter for excitation, optical isolators 9 and 11, amplification section 10 comprising a rare-earth-element-doped optical fiber, and optical filter 12. This arrangement indicates the problem that a plurality of optical components and control circuits such as optical coupler 1, dummy signal semiconductor laser emitter 2, dummy signal semiconductor laser emission control circuit 3, light-receiving element 4, and optical beam splitter 5, must be added to the EDFA.
Generally, the optical transmission system requires a high reliability. To increase the number of components and circuits used in the system mostly degrades the reliability. That is, the system reliability is generally estimated by adding values of the reliability parameter (called the failure rate or the FIT number) with respect to individual components in the system arrangement. Therefore, increase of the components directly causes decrease of the (total) reliability of the system. In particular, the reliability parameter applied to an optically active component such as a laser emitter is lower than that of a passive component such as an optical coupler by the order of one or two digits. Therefore, in order to improve the system reliability, an additional design for making a constitution of multi-components and multi-circuits is necessary, and simultaneously, the system arrangement and the estimate of the reliability themselves become complicated (refer to, for example, J. Schesser, et al., AT & T Technical Journal, p. 16, January/February, 1995).
Accordingly, in order to depress the optical surge generated by the EDFA in the optical transmission system, it is preferable to use as small a number of components as possible by selecting components which can depress the optical surge, so that the conventional method which demands addition of many components (including active components) and circuits is unnecessary. It is furthermore preferable that only optic--optic interactive components, which need no electric control circuit, be added.
The following are known methods for simply attenuating the transmitted optical signal: cladding the surface of the optical waveguide and using a calcite or glass waveguide. However, in these conventional methods, it is difficult to accurately control the optical attenuation by a desired amount, and so the necessary optical signal as well as the unnecessary high-intensity signal is also attenuated.
On the other hand, an optical fuse type, which uses a burnout phenomenon due to high-intensity light input into an end face of a compound semiconductor optical waveguide, disclosed in Japanese Patent Application, First Publication, No. Hei 9-146056, cannot be used plural times because an irreversible phenomenon such as the burnout is used.